As many women have annual mammograms as part of screening for breast cancer, many of them have multiple older mammograms available to compare with the current mammogram. In a high volume, rapid screening program, it is not practical for a radiologist to compare all of the prior mammograms to the current examination. We are not aware of any comprehensive study that investigates the diagnostic performance as a function of the reference examination nor is a there an age dependent optimal reference (e.g. 2 year prior for women age 55 and over or l year prior for women under 55 years of age). In addition to the possibility of an age dependent optimal reference, there may be a difference in an age dependent optimal reference in women who take hormone replacement therapy and those who do not. In this study, we will ask seven radiologists to diagnose 180 cases under four different reading modes similar to a screening environment. Except the first mode that does not include a comparison to the prior examinations, all other three modes will include a prior mammogram for comparison with the current examination. The time interval between two examinations will be either one, two, or three years. Hence, using an R0C-type methodology, we will investigate how these environments affect (or not) reader-performance, and we hope to find an effective and efficient reading mode for the majority of patients. Alternatively, we may find that different modes are optimal for different patients (e.g., for different age groups).